Comparison of Biodiesel Production by a Supercritical Methanol

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Comparison of biodiesel production by a supercritical methanol method from microalgae oil using solvent extraction and hydrothermal liquefaction processes Chihiro Fushimi, and Akihito Umeda Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00904 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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Research Paper submitted to Energy &Fuels, special issue "In Honor of Michael J. Antal" (Revised

version 3)

“Comparison of biodiesel production by a supercritical methanol method from microalgae oil using solvent extraction and hydrothermal liquefaction processes”

Chihiro Fushimi*#, Akihito Umeda# Department of Chemical Engineering, Tokyo University of Agriculture and Technology 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan

*Corresponding author: Tel/Fax: +81-42-388-7062, e-mail: [email protected] # Chihiro Fushimi and Akihito Umeda equally contributed to this study.

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Abstract The production of biodiesel from bio-oils using two different processes was investigated. The bio-oils were either produced via freeze-drying followed by extraction with chloroform/methanol solvents (solvent-extracted oil) or by employing a subcritical (573 K, 10 MPa, 30 min) hydrothermal liquefaction process (HTL oil). Both oils were derived from the diatome Fistulifera solaris JPCC DA0580 and were converted to fatty acid methyl esters (FAME) via transesterification/esterification reactions with supercritical methanol (SCM) at relatively mild conditions (593 K, 13 MPa). The impact of reaction time (10−60 min), methanol:oil molar ratio (42:1 and 21:1), and water content (0−5wt% based on the weight of the oil) on the FAME yield was investigated. FAME yields from HTL oil (75%–80% after 30 min) were higher than corresponding experiments with solvent-extracted oil (48%–64% after 30 min) under all investigated conditions. This was attributed to the HTL oil containing more free fatty acids (FFA), which SCM promotes the conversion of this oil to FAME. Decreasing the ratio of methanol:oil had little impact on the FAME yield from the HTL oil. Adding up to 2.5wt% water (based on the weight of the oil) to the oil–SCM mixture also had little impact on the FAME yield from either oil. However, adding 5.0wt% water decreased the FAME yield from both oils, particularly from the solvent-extracted oil.

Keywords: microalgae, supercritical methanol, fatty acid methyl ester (FAME), solvent-extracted oil, hydrothermal liquefaction oil 2


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1. Introduction The production of biofuels and biochemicals from microalgae has gained increasing amounts of attention in recent years because of microalgae’s higher growth rate and higher photosynthetic efficiency than those of oil crops—such as Jatropha, coconut, rapeseed and oil palm—per unit area.1−7 The production of biodiesel fuel (BDF) from microalgae requires the following operations: culturing; harvesting/dewatering (or concentration) of algae; extraction of bio-oil (lipids); conversion of the bio-oil into BDF; and separation/purification of the products. The bio-oil is usually extracted using organic solvents such as n-hexane and chloroform.4,8,9 This also requires a drying process prior to extraction and recovery of the solvent after extraction, both of which are energy-intensive.10,11 For economic viability and environmental sustainability, the development of large-scale BDF production requires significant decreases in energy use. Hydrothermal liquefaction (HTL) using subcritical water (ca. 473–623 K, 5–20 MPa) is an alternative method to produce bio-oil from microalgae. HTL does not require the energy-intensive drying and solvent-recovery processes, and its bio-oil yield is usually higher than conventional solvent extraction methods because in HTL mixture of the components derived from not only lipids but also hydrocarbons and proteins in the microalgae is obtained as HTL oil.4,9,12–14 In addition, the presence of subcritical water has been shown to hydrolyzed triacylglycerides (TG) contained in microalgal (Chlorella vulgaris) lipids, producing 3


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diacylglycerides (DG), monoaclyglycerides (MG) and fatty acids (FA).15 The production of BDF, which is mainly composed of fatty acid methyl ester (FAME), usually involves a transesterification reaction between oil and methanol that is facilitated by alkali (NaOH, KOH, or CaO) or acid catalysts (H2SO4, WO3).16−19 BDF can also be produced by a supercritical methanol (SCM) method, which was originally developed by Saka and colleagues.20−24 Here, SCM (> 512 K and > 8.1 MPa) is used as the reactant and a relatively large amount of FAME is produced in a short reaction time without either using a catalyst or producing wastewater. In addition, FA that is present in the oil can be converted into FAME by esterification without producing soap. Water also does not inhibit the production of BDF.17 The same group later developed a two-step method that combines hydrolysis of TG in subcritical water and esterification of the hydrolyzed products in near-SCM under relatively mild conditions (543 K and 7 MPa). Using lower temperatures was found to avoid the decomposition and isomerization of produced FAME that would occur at high temperatures, and decreased the reactor cost.17,25,26 Based on these results, a combination of HTL and transesterification/esterification by the SCM method appears promising for large-scale BDF (FAME) production from microalgae. To date, many studies have been conducted on the transesterification of biomass-derived oil to produce FAME in SCM,20,21,26−39 as summarized in Table 1. However, few studies have investigated the conversion of microalgal-derived bio-oil into FAME via the SCM method. In 4


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particular, to the best of our knowledge, no study has been reported on the reactivity of microalgal-derived oil processed by HTL (hereafter, ‘HTL oil’) to produce FAME using SCM. Relatedly, Nan et al.39 recently investigated the production of microalgal-derived oil obtained by solvent extraction (hereafter, ‘solvent-extracted oil’) from transesterification reactions in SCM and supercritical ethanol (SCE). They reported an FAME yield of 91% under conditions of 593 K, 15.2 MPa, a methanol:bio-oil ratio of 19:1 and 7.5 wt% water. Patil et al.40 and Reddy et al.41 studied the direct transesterification of wet algae individually in SCM and SCE, respectively, to simplify the overall system. However, because methanol and ethanol are mixed with the microalgal residue, a drawback of both of these methods is the difficulty in recovering microalgal nutrients for future culturing processes or recycling microalgal residue to be used for anaerobic digestion or animal/fish feed. The objective of this study is to investigate the reactivity (in terms of FAME production via the SCM method) of solvent-extracted oil and HTL oil derived from microalgae via relatively mild transesterification/esterification reactions. 2. Experimental 2.1 Microalgae A marine diatom, Fistulifera solaris JPCC DA0580,42 was pre-cultured in a flat 1.5 L culture vessel containing 2-fold concentrated f medium (2f medium),43 which was dissolved 5


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in artificial seawater (MARINE ART® SF-1, Osakayakken.Co.Ltd.44), for a period of 7 days. The diatom was then cultured for 12 days in a 10 L flat-panel photobioreactor, again using the 2f medium.45,46 The compositions of the 2f medium and the artificial seawater are shown in Table 2. The algal concentration was measured by monitoring changes in the sample’s mass. Further details of the procedure are available in our previous study.47 2.2 Oil extraction methods 2.2.1 Extraction with organic solvent Solvent-extracted oil was obtained according to a method reported elsewhere.48 Two solvents of different chloroform:methanol ratios (solvent A: 20:1 v/v; Solvent B: 2:1 v/v) were used for the extraction. Freeze-dried microalgae were ground and mixed with solvent A before being shaken in an ultrasonic shaker for 30 min. Distilled water was then added to the solution and the mixture (solution I) was centrifuged. The top phase (I-1, consisting of methanol/water/algal residue) was separated from the bottom phase (I-2: chloroform/algal oil) with a nylon mesh. I-2 was collected in a vial while I-1 was mixed with solvent B to form solution II, which was also shaken and centrifuged. The bottom phase (II-2: chloroform/algal oil) was separated from the top phase (II-1: methanol/water/algal residue phase) with a nylon mesh and then added to the vial containing I-2. The vial’s contents (I-2 and II-2) were dried in a 353 K nitrogen atmosphere to 6


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evaporate residual chloroform. The dried oil was then dissolved with hexane and filtered. The solvent-extracted oil was obtained following evaporation of hexane under the same conditions used to evaporate the residual chloroform. 2.2.2 Hydrothermal liquefaction (HTL) After culturing, the culture fluid was centrifuged. The concentration of the microalgal slurry was 30-g-dry-algae/L-slurry. 100 mL of the microalgal slurry was loaded into an autoclave HTL reactor (TPR-1, 300 mL, Taiatsu Techno® Corporation, Osaka, Japan). The reactor was sealed and purged with 2.5 MPa Ar gas. An electric heater then increased the reactor temperature to 573 K. The combination of the Ar gas pressure and saturated vapor pressure of water gave an overall pressure of 10 MPa in the reactor. The HTL reaction was then carried out this temperature and pressure for 30 min. After the reactor was cooled, the water-soluble fraction was separated from the bio-oil and solid products by filtration. Residual bio-oil attached to the solid products was extracted by immersion in acetone at room temperature for 60 min with the solid product and bio-oil then separated by filtration using a nylon mesh. Acetone was evaporated in a 353 K nitrogen atmosphere. The bio-oil was then mixed with hexane and filtered. HTL oil was obtained by evaporating hexane using the above evaporation conditions. Further details of the HTL and bio-oil recovery processes are available in our previous study.47 2.3 Analyses of solvent-extracted and HTL oils 7


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The amount of FAs in the solvent-extracted oil and HTL oil was measured by converting them into FAME using the acid catalyst method. Here, approximately 30 mg of the each oil was mixed with 5 mL of HCl/methanol and shaken to form mixture 1, which was then heated at 373 K for 90 min. After cooling to room temperature, hexane and distilled water were added to form mixture 2, which was then shaken and centrifuged. The top (hexane) phase was separated and mixed with distilled water to form mixture 3, which was also shaken and centrifuged. The top (hexane) phase was again recovered and a small amount of Na2SO4 was added to facilitate further dehydration (solids were then filtered). FAME was obtained after hexane was evaporated in a 353 K nitrogen atmosphere. The produced FAME was dissolved in dichloromethane (purity 99.5%, Kanto Chemicals Co. Ltd.) and measured using a gas chromatograph (GC, GC 2014, Shimadzu Corporation, Kyoto, Japan) equipped with a flame ionization detector (FID) and a ZB-5ht column (Phenomenex), using He as the carrier gas. FIM-FAME-7 (Matreya LLC) was used as the standard FAME sample for the GC-FID measurement. 2.4 Conversion of solvent-extracted and HTL oils into FAME via the SCM method Figure 1 shows the experimental apparatus for the conversion using the SCM method. A defined ratio of oil (solvent-extracted or HTL) and methanol (purity 99.8%, Kanto Chemical Co. Ltd.) was poured into a 2.28 mL stainless steel reactor tube. The reactor size was limited by the amount of bio-oil produced by each batch of culturing. The pressure inside 8


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the reactor tube was measured with a pressure gauge (EN837-1, Swagelok). The initial pressure was increased to approximately 3.4 MPa by adding nitrogen gas before the reactor was sealed. A bath (i.d. = 139.8 mm) of silica sand was fluidized using air at a flow rate of 35 NL/min (0.038 m/s @STP) and then heated to, and maintained at, 593 K using an electric furnace, a type-K thermocouple and a temperature controller (DB650, CHINO corporation). The reactor tube was then immersed in the fluidized sand bath, and then the oil and methanol reacted. The reaction temperature was approximated by another type-K thermocouple that measured the temperature of the outside of the reactor tube. A preliminary test (data not shown) confirmed that pressure rapidly increased in the first 2 min and reached a plateau after 5 min. The onset of the reaction was thus defined as 5 min after the tube was immersed in the fluidized bed. Once reaction conditions (593 K, 13 MPa) had been maintained for the desired reaction time (varied between 10 and 60 min), the reactor was immersed in a water bath at room temperature to quench the reaction. Two methanol:oil molar ratios (42:1 and 21:1) were investigated. After degassing the nitrogen gas, reaction products were recovered from the reactor tube by adding 2 mL hexane three times and 3 mL distilled water. The mixture was poured into another vessel and vortexed followed by centrifuging at 9600 rpm for 6 min and separation. The top (hexane-soluble) phase was mixed with 2 mL distilled water and 9


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centrifuged/separated twice more. The ultimate top (hexane-soluble) phase was recovered and a small amount (ca. 5 mg) of Na2SO4 was added to complete the dehydration by filtering. FAME was obtained once hexane was evaporated in a 353 K nitrogen atmosphere. The produced FAME sample was analyzed using the GC-FID method described above. 3. Results and discussion 3.1 Oil yields Table 3 shows the yield of oil for the various experiments, as defined by Eq. 1. : [ %] =

[] []

× 100

(1)

Oil production via the two methods was repeated several times (seven for solvent-extracted oil and five for HTL oil) to confirm repeatability. The average oil yields for the solvent-extracted and HTL methods were 45.7 and 43.5wt%, respectively. The higher yield for the solvent extraction method was attributed to destruction of the microalgal cell walls during the freeze-drying and grinding processes. However, the similarity between the two yields suggested that the HTL method was effective for the production of bio-oil. The FA analyses of the solvent-extracted and HTL oils shown in Figure 2 illustrate that the FA—responsible for 59 and 47wt%, respectively—in both mainly consisted of palmitic acid (C16:0) and palmitoleic acid (C16:1). In the present FA analysis, only neutral lipids in the oil were converted to FAME by transesterification with acid catalyst and were able to be measured with the GC-FID. The remaining, undetected, oil was considered to have 10


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been derived from glycolipids and phospholipids in the microalgae. Liang et al.49 reported that the FAME produced from the dried biomass of Fistulifera sp. was dominated by myristic acid (C14:0), C16:0, C16:1, and icosapentaenoic acid (C20:5), which, apart from the C20:5 species, is in broad agreement with the present findings. HTL oil contained less FA with no eicosenoic acid (C20:1) detected. Guo et al.50 reported that FA yields were approximately 10wt% after the HTL reaction of microalgae at 643 K and 22 MPa for 60 min. We attributed the differences observed between our results and those of Guo et al. to the decomposition of FA in subcritical water during the HTL reaction. 3.2 Production of FAME by SCM method 3.2.1 Effect of reaction time on FAME yield Various reaction times were investigated for the two methods, while other parameters held constant (593 K, 13 MPa, methanol:oil = 42:1). Each experiment was repeated two or three times to confirm reproducibility, with the average values then taken. Figure 3 shows the average yield of FAME for the various experiments, as defined by Eq. 2, as a function of reaction time. FAME yield [wt%] =

,-./0 ,1

× 100

(2)

where MFAME is mass of produced FAME after conversion reaction in SCM [g] and MT is the total mass of FA, which could be converted into FAME, in algal oil [g]. For the HTL oil, the FAME yield increased during the first 60 min; reaching 80% 11


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after 30 min and 83% after 60 min. Longer reaction times at temperatures above 573 K have been reported to decrease FAME yields because of thermal decomposition and/or isomerization reactions in SCM.32,38,51,52 However, FAME derived from HTL oil was found to stable in SCM. In contrast, for the solvent-extracted oil, the FAME yield was 64% after 30 min but decreased to 47% after 60 min. The yield of FAME from the HTL oil was always higher than that from the solvent-extracted oil, indicating higher reactivity of the HTL oil in SCM. For solvent-extracted oil, the optimum reaction time was clearly 30 min. For HTL oil, when the reaction time was increased from 30 to 60 min there was a small increase in the FAME yield (i.e. from 80% to 83%). Changi et al.53,54 reported hydrolysis of ethyl oleate (i.e. representative of fatty acid ethyl ester) is catalyzed by oleic acid and H+ derived from oleic acid and water. In this study, because water in solvent-extracted oil is completely evaporated before transesterification reaction in SCM and FA concentration is considered to be higher in HTL oil than in solvent-extracted oil, it is unlikely that FAME produced from solvent-extracted oils is more prone to be hydrolyzed than FAME produced from HTL oils. Thus, thermal decomposition and/or isomerization of produced FAME are considered to take place after 30 min. We believe the difference of net FAME yield between the HTL oils and solvent-extracted oils is due to the kinetic reason. Warabi et al.22 reported that esterification of FA is faster than transesterification of TG, DG and MG and that reaction rate of transesterification of MG into 12


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FAME is smallest. In the case of solvent-extracted oil, prolonged time from 30 to 60 min promotes transesterification of TG and DG into MG, producing FAME. However, the reaction rate of this transesterification reaction is probably smaller than that of thermal decomposition and/or isomerization of produced FAME. Thus, the net FAME yield from solvent-extracted oil decreased after 60 min. In contrast, HTL contains larger amount of FA and the reaction rate of esterification of FA is comparable to that of thermal decomposition and/or isomerization of FAME. Thus, the net FAME yield is same after 30 and 60 min. In consideration of the economics of the process, a 30-min reaction time was selected in the following experiment.

3.2.2 Effect of methanol:oil ratio Figure 4 shows the effect of varying the methanol:oil ratio on FAME yield while other conditions were held constant (593 K, 13 MPa and 30 min). Each reaction was repeated three times to confirm reproducibility. For extraction both methods, a higher amount of methanol increased the FAME yield, in agreement with work reported elsewhere.34 However, the impact of halving the methanol ratio was more pronounced for solvent-extracted oil (which decreased from 64% to 48%) than for HTL oil (from 80% to 75%) though the HTL oil yield remained higher. As noted above, for both oils the majority of the FAME consisted of methyl palmitate (C16:0) and methyl palmitoliate (C16:1(cis-9)), 13


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which also agreed with previous findings.49 The yield of methyl palmitoliate (C16:1(cis-9)) was far more sensitive to changes in the ratio than the yield of methyl palmitate (C16:0), which was largely unaffected in both HTL and solvent-extracted oils. Jazzar et al.55 studied the sensitivity of FAME yield from various fatty acids including methyl palmitoliate to molar ratio of methanol. They reported similar sensitivity of methyl palmitate and methyl palmitoliate production to the ratio of methanol. Detailed study is needed to investigate the sensitivity and reactivity of methyl palmitate and methyl palmitoliate at various molar ratio of methanol. The solvent-extracted oil contained approximately 5 wt% of eicosenoic acid (C20:0); though this could not be converted to methyl eicosenate because, as Choi et al. reported,32 more severe conditions are required to convert longer-chain TG into FAME. 3.2.3 Effect of water on FAME yield To investigate whether adding water affected the FAME yield, distilled water (2.5 or 5.0 wt% of the weight of oil) was added to the methanol–oil mixture prior to pressurizing with N2 gas. As the results in Figure 5 show, while 2.5wt% water had no effect on FAME yield for either type of oil, adding 5.0wt% water significantly decreased the FAME yields (from 65% to 56% and from 77% to 71% for solvent-extracted and HTL oil, respectively). Adding 5.0wt% water clearly inhibited the transesterification and esterification reactions in SCM and this was more marked for the solvent-extracted oil. In previous studies, it was reported water (≥5%) decreased FAME yield in SCM56 and fatty acid ethyl ester (FAEE) 14


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yields in SCE.57 Though detailed mechanism of the decrease of FAME yields by addition of 5.0wt% was unclear, there are three possible reasons (1) kinetic effect due to dilution of reactants by water, (2) decrease in equilibrium conversion and (3) change in phase equilibrium as Pinnat and Savage reported.57 However, it is difficult to specify each reason affects. 3.3 Reactivity of solvent-extracted oil versus that of HTL Oil in SCM In terms of producing FAME, HTL oil was more reactive in SCM than solvent-extracted oil for all reaction conditions tested. Two possible reasons could explain this: 1. During the HTL process, some of the lipids (TG) in microalgae could have been hydrolyzed to form free fatty acids (FFA), producing a mixture of partially hydrolyzed lipids—including FFA, DG and MG15, 58, 59—while only lipids (TG) were produced in the solvent-extracted oil. During transesterification, TG are converted (in series) to DG and MG to produce FAME and glycerol. However, the stability of MG limits their reactivity in the transesterification reaction.22,23 By contrast, the esterification reaction between FFA and methanol is relatively fast, explaining why HTL oil could more easily be converted into FAME than solvent-extracted oil. The decrease of FAME yield between 30 and 60 min (Figure 3) suggests that the thermal decomposition occurred more rapidly than transesterification of the MG. 15


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2. SCM catalyzes the esterification reaction (the conversion of FFA to FAME) rather than transesterification reaction (the conversion of TG, DG and MG to FAME) in a similar fashion to acid catalysts.23 We surmise that the conversion of HTL oil—which contains more FFA—to FAME is more significantly facilitated by SCM’s catalytic effect than that of solvent-extracted oil, which contains more TG. 4. Conclusions The FAME yield from oil derived via solvent extraction and HTL processes from the diatome Fistulifera solaris JPCC DA0580 using transesterification/esterification reactions with SCM was studied at 593 K and 13 MPa in a small batch reactor. The following conclusions were drawn: (1) FAME yields from HTL oil (75%–80% after 30 min) were higher than those from solvent-extracted oil (48%–64%) under all investigated reaction conditions. This was attributed to HTL oil containing more FFA than solvent-extracted oil and because SCM promotes FFA-esterification. (2) FAME yields from HTL oils increased when the reaction time was increased to 60 min, suggesting that 1) the longer exposure did not lead to thermal decomposition and/or isomerization of FAME or 2) the reaction rates of the thermal decomposition and isomerization balance those of esterification of FA.

16


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(3) An optimum reaction time of 30 min is suggested to avoid unwanted thermal decomposition of FAME produced from solvent-extracted oil. (4) Decreasing the methanol:oil ratio from 42:1: to 21:1 significantly decreased the FAME yield from solvent-extracted oil (from 64% to 48%) and slightly decreased the yield from HTL oil (from 80% to 75%). Mehyl palmitoriate (C16:1 (cis-9)) is more sensitive to molar ratio of methanol than other FAME. (5) Adding up to 2.5wt% of water had no impact on FAME production in SCM. However, adding 5.0wt% water decreased the FAME yield from HTL oil and, especially, from solvent-extracted oil. Acknowledgments This study was supported by a Grant-in-Aid for Scientific Research B (Kakenhi Kiban B, 26289302) and the Center of Low Carbon Society Strategy, Japan Science and Technology Agency (JST-LCS). The authors thank Dr. Yue Liang (Tokyo University of Agriculture and Technology, TUAT) for technical support with the analysis of the oil. The authors are grateful to Mr. Ryo Tomita and Ms. Chiemi Tachibana (TUAT) for their useful discussion for revising this paper. The authors also appreciate technical support during the culturing of the microalgae samples by Professor Tsuyoshi Tanaka, Professor Tomoko Yoshino, Dr. Yoshiaki Maeda, Dr. Daisuke Nojima and Dr. Masaki Muto (TUAT).

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22


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Energy & Fuels

Table 1 Summary of operating conditions, feed oil and FAME yields using SCM method.

23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

methanol:oil

Reaction

molar ratio

time

Page 24 of 32

FAME

Water

yield

content

[%]

[%]

Rapeseed

95

−

21

4

Rapeseed

95

−

20

21:1

4

Rapeseed

80

−

20

20

40:1

5

Sunflower

97

−

26

623

20

42:1

7−15

Coconut

95

−

27

623

20

42:1

30

Soybean

95

−

28

673

40

3:1

20

Olive oil

76

−

29

673

41

6:1

6

Chicken fat

88

−

30

623

10−20

9:1

8−10

Chicken fat

67, 80

−

31

623

35

40:1

20

Palm olein

95

−

32

543

10

1:1 (wt:wt)

45

Canola oil

96

−

33

533

16

40:1

10

Krating oil

90

−

34

593

15

40:1

5

Jatropha oil

85

−

35

573

20

40:1

1 mL/min

Soybean fried oil

82

10

35

645

15−25

42:1

16

Palm oil

82

−

36

598

35

42:1

60

Soybean fried oil

84

−

37

573

32

40:1

25

Vegetable oil

80

−

38

573

32

18:1

25

Vegetable oil

70

−

38

593

16

40:1

25

Vegetable oil

60

−

38

593

15

19:1

31

593

13

42:1

30

Microalgae (HTL oil)

80

−

This study

593

13

21:1

30

Microalgae (HTL oil)

75

−

This study

593

13

42:1

30

T

P

[K]

[MPa]

623

45

42:1

4

623

19

42:1

623

19

673

Oil type

[min]

Microalgae (solvent-extracted oil)

Microalgae (solvent-extracted oil)

24


91

64

7.5

−

Ref.

39

This study

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Energy & Fuels

Table 2 Compositions of (a) 2f medium43,45,46 and (b) artificial seawater44 (a) 2f medium NaNO3 NaH2PO4·2H2O Vitamin B12 Biotin Thiamine HCl Na2SiO3·9H2O f/2 metals Distilled water

f/2 metals 30 mg 2.4 mg

Na2EDTA·2H2O FeCl3·6H2O

440 mg 316 mg

0.2 µg 0.2 µg 40 µg 4 mg 0.4 mL 99.9 mL

CoSO4·7H2O

1.2 mg

ZnSO4·7H2O MnCl2·4H2O

2.1 mg 18 mg

CuSO4·5H2O Na2MoO4·2H2O Distilled water

0.7 mg 0.7 mg 100 mL

(b) Artificial seawater NaCl

22.1 g

MgCl2·6H2O

9.9 g

CaCl2·2H2O KCl KBr SrCl2 LiCl MnCl2·4H2O

1.5 g 0.61 g 96 mg 13 mg 1 mg

Na2SO4 NaHCO3 Na2B4O7·10H2O NaF KI

3.9 g 0.19 g

0.6 µg 8 µg

CoCl2·6H2O FeCl3·6H2O (NH4)6Mo7O24·4H2O Distilled water

AlCl3·6H2O Na2WO4·6H2O

2 µg

25


78 mg 3 mg 81 µg 2 µg 5 µg 18 2 µg 25 L

Energy & Fuels

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Page 26 of 32

Table 3 Oil-extraction ratio for solvent-extracted and HTL oils Extraction method Oil yield [wt%]*

Extraction method

Oil yield [wt%]*

Solvent 1

36.4

HTL 1

33.7

Solvent 2

45.1

HTL 2

47.3

Solvent 3

52.4

HTL 3

42.7

Solvent 4

34.4

HTL 4

49.9

Solvent 5

55.5

HTL 5

43.9

Solvent 6

46.0

Solvent 7

50.5

Average

45.8±7.4

43.5±5.5

*based on dry microalgae mass

26


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Energy & Fuels

Figure 1 Experimental apparatus for the conversion of bio-oil using supercritical methanol. 281x160mm (96 x 96 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 Fatty acid compositions of solvent-extracted and HTL oils. 240x175mm (96 x 96 DPI)


Page 28 of 32

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Energy & Fuels

Figure 3 Effect of reaction time in the SCM method on FAME yield (593 K, 13 MPa, methanol: oil=42:1 mol/mol) 230x175mm (96 x 96 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 Effect of molar methanol:oil ratio in the SCM method on FAME yield (593 K, 13 MPa, 30 min). C16:0, C16:1 and C14:0 represent methyl palmitate, methyl palmitoliate, and methyl myristate, respectively. ‘Others’ includes methyl stearate (C18:0), methyl oleate (C18:1(trans-9)), methyl octadeca9,12-dienoate (C18:2(all cic-9,12), and methyl eicosenate (C20:0). 224x156mm (96 x 96 DPI)


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Energy & Fuels

Figure 5 Effect of water content in the SCM method on FAME yield (593 K, 13 MPa, 30 min, methanol:HTL oil=21:1 mol/mol, methanol:solvent extracted oil=42:1 mol/mol). 233x175mm (96 x 96 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 Summary of the formation of FAME from microalgae via HTL and solvent-extraction processes. 233x142mm (96 x 96 DPI)


Page 32 of 32

Comparison of Biodiesel Production by a Supercritical Methanol

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